The invention relates to a method of shaping an aspherical optical element such as a lens or a mirror. The invention applies in particular to shaping segments of segment mirrors of large dimensions.
The shaping of aspherical optical elements is a process that is lengthy, difficult, and expensive, in particular when it is necessary to obtain complex aspherical surfaces of large dimensions (e.g. of diameter greater than or equal to 1 meter (m), with a shape that is defined very accurately with a very low level of roughness.
These conditions are to be found, for example, when fabricating segments for the purpose of building up very large segmented mirrors, that may reach diameters of several tens of meters and that are used as the main mirrors of the most recent astronomical telescopes known as “Extremely Large Telescopes” (ELT). These mirrors are made up of a large number (several hundreds or thousands) of segments, that are generally hexagonal in shape and that are assembled together edge to edge. The position and the orientation of each segment can be controlled by actuators with accuracy and resolution of nanometer order. Each segment is constituted by an off-axis aspherical mirror of relatively large dimensions (of the order of one meter or more).
A first technique that can be used for shaping such aspherical optical elements is numerically controlled grinding and polishing using tools of small dimensions mounted on robot arms. That method has been used in particular for fabricating the elements of the “Gran Telescopio Canarias” telescope: see the article by R. Geyl et al.: “Gran Telescopio optics manufacture: final report”, Proc. SPIE, Vol. 6273, 2006. That method presents the drawback of giving rise to numerous surface defects of high spatial frequencies, defects that need to be corrected in a subsequent step of machining by ion bombardment. This results in fabrication that is lengthy and expensive.
A second technique known as stress mirror polishing (SMP) was proposed in 1980 by J. Lubliner and J. Nelson. That technique, used for the American Keck telescope and recommended for the future 30-meter telescope (TMT) makes it possible rapidly to obtain optical surfaces that are complex, and highly aspherical, while being reduced merely to simple spherical polishing. In that technique, a blank for shaping having an optical surface of plane of spherical shape is deformed mechanically so as to present an aspherical shape that is complementary to the shape desired. Thereafter, the deformed optical surface is shaped by abrasion (it is ground and polished) so as to make it spherical. Finally, the blank is allowed to relax and return to its initial rest state. The optical surface that acquired a spherical profile while the blank was in its deformed state, thus takes on the desired aspherical shape.
The SMP technique is faster than numerically controlled polishing with small tools and it enables surfaces to be obtained that are of better quality. Nevertheless, the deformation stresses applied to the blank are not completely uniform, and that gives rise to defects at the periphery of the shaped optical surface. To remedy that drawback, it is possible to use a blank of diameter greater than that of the desired optical element, so as to make it possible to cut off a peripheral ring that corresponds to the zone in which such defects are concentrated, which zone has a width of the same order as the thickness of the blank (typically several tens of millimeters). That increases the cost of the finished optical element.
In a variant, or in addition, it is possible to correct surface defects by a subsequent step of machining by ion bombardment. However that operation also leads to extra expense.
The SMP technique is described in detail in the following articles:                J. Lubliner, J. Nelson: “Stressed mirror polishing. 1: A technique for producing non-axisymmetrical mirrors”, Applied Optics, Vol. 19, No. 14, pp. 2332-2340, 1980;        J. Nelson et al.: “Stressed mirror polishing. 2: Fabrication of an off-axis section of a paraboloid”, Applied Optics, Vol. 19, No. 14, pp. 2341-2352, 1980; and        F. S. Sporer: “TMT—Stressed mirror polishing fixture study”, Proc. SPIE, Vol. 6267, 2006.        